水平浮射流排放断面不对称性的发生和作用机理研究

In recent years, the coastal pollution in China has not been alleviated, and rather, it has become worse in certain region, and the major pollution source comes from ocean dispoal of wastewater from land-based sources. Wastewater is typically discharged in the form of turbulent buoyant jet into the sea, and integral models that are based on axisymmetric velocity and concentration distributions are widely used to simulate the centerline trajectory and dilution process. However, the unstable stratification between the jet and the receiving water will result in non-axisymmetry, when the jet initial Reynolds number and densimetric Froude number are small, a flow bifurcation may even occur, significantly affecting the development of the jet trajectory as well as its dilution process and nullifying the applicability of traditional integral models. The proposed study will focus on the typical case of horizontal buoyant jet, of which the direction of the initial velocity is orthogonal to that of the buoyancy force. Flow visualization, particle image velocimetry and laser induced fluorescence will be employed to investigate the flow pattern, velocity and concentration fields at the jet center plane of symmetry as well as a series of lateral cross sections. Based on the experimental results, we can obtain the parameter domain for the occurence of the bifurcation as well as its mechanism, derive velocity and concentration distributions after accounting for non-axisymmetry, and incorporate it in the integral model to accurately simulate the jet trajectory and dilution process. The research output can be applied in the outfall design and mixing zone analysis of horizontal buoyant jets, and at the same time provide theoretical references for the parameter selection of future physical models of marine discharge.

我国近海环境的污染状况近年来不仅没有得到明显好转，局部甚至出现恶化，其中主要的污染源是陆源入海排污口排污。污水通常采用紊动浮射流的方式排放入海，而基于断面流速和浓度分布关于中心线对称的积分模型则广泛用于模拟浮射流中心线轨迹和稀释过程。然而浮射流与受纳水体的不稳定分层会引起断面不对称，当射流初始雷诺数和密差弗汝德数较小时，还会发生流动分叉，对射流轨迹发展和稀释过程产生显著影响，造成传统积分模型失效。本项目选取初始速度与浮力方向正交的水平浮射流这一典型情况，应用流动显示、粒子图像测速和激光诱导荧光技术考察射流中心对称面和一系列沿程断面的流动形态、速度场和浓度场。通过实验得到发生射流分叉的参数域和发生机理，建立考虑不对称性的断面速度和浓度分布表达式,并引入积分模型精细模拟射流轨迹和稀释过程。研究成果将应用于水平浮射流的排放设计和混合区分析，同时也为未来排海工程的物理模型参数选取提供理论依据。

我国近海环境的污染状况近年来不仅没有得到明显好转，局部甚至出现恶化，其中主要的污染源是陆源入海排污口排污。污水通常采用紊动浮射流的方式排放入海，而基于断面流速和浓度分布关于中心线对称的积分模型则广泛用于模拟浮射流中心线轨迹和稀释过程。然而浮射流与受纳水体的不稳定分层会引起断面不对称，当射流初始雷诺数和密差弗汝德数较小时，还会发生流动分叉，对射流轨迹发展和稀释过程产生显著影响，造成传统积分模型失效。本项目采用PLIF技术，在前人提出的流动分叉Re-Fr参数域内开展流动显示实验，结果表明，在前人提出的流动分叉Re-Fr参数域并没有出现严格的流动分叉，即从主射流分离发展出的一股完全独立的流动，只出现了近似流动分叉的拟二次射流。同时，流动显示捕捉到一系列不稳定流动现象，包括下沉羽流、拟二次射流和流动剥离现象。根据流动特征的不同划分了三种主要流动形态，并确定了三种流态在Re-Fr参数域的分布。同时采用经过灰度-浓度率定的PLIF实验，定量的观测了中低雷诺数下的负浮力射流中心对称面上的浓度分布，并分析了沿射流中心线的浓度衰减、各下游断面的浓度分布及射流扩展情况，并研究了Re和Fr对浓度衰减和射流扩展的影响。在实验观测的基础上，本项目构建了适用于任意（正、负）浮力的倾斜浮射流积分模型和基于不对称断面假设的水平浮射流积分模型，与实验观测结果的对比表明，模型对于负浮力射流及不对称性明显的射流段较基于卷吸系数经验公式和断面对称假设的传统积分模型具有更好的模拟效果。研究成果将应用于水平浮射流的排放设计和混合区分析，同时也为未来排海工程的物理模型参数选取提供理论依据。此外，本项目的实验数据和理论分析也将为浮力射流机理研究提供有益补充。